Many modern chemical manufacturing processes require separating solids from a mixture of solids and gases. For example, when a fluidized bed of solid catalyst and feedstock is reacted to form a reaction mixture of spent catalyst and a vapor-phase product, the catalyst typically must be separated from the mixture when the reaction is complete. A common example of such a process is the catalytic cracking of relatively high molecular weight hydrocarbon feedstock to lower molecular weight gasoline range materials.
Modern catalytic cracking operations react a mixture of hot solid catalyst with a relatively high molecular weight feedstock inside a riser reactor. In this type of operation, a fluidized bed of hot catalyst and feedstock is converted to lower molecular weight hydrocarbon vapors and spent catalyst as material moves upwardly through the reactor. The spent catalyst then must be separated from the hydrocarbon vapors so that the spent catalyst can be regenerated and reused and so that the cracked vapors can be further processed.
Because catalytic cracking and the subsequent catalyst-vapor separation typically are carried out at temperatures around 1000 degrees Fahrenheit or greater, gasoline range products produced by the catalytic cracking reaction are subject to undesired secondary thermal cracking if they are not quickly separated from spent catalyst and moved to a lower temperature environment. For example, it is believed that about ten percent of the desired gasoline range reaction products can be lost if the reaction products are maintained in an 1100 degree Fahrenheit environment for 4 to 5 seconds. Therefore, the time spent separating catalyst from gasoline range vapors can substantially impact product yields.
In some refineries, catalytic cracking and catalyst separators have been carried out in open vapor path systems having a riser reactor located within a large catalyst disengagement vessel. In this type of system, catalyst and cracked vapors exiting the riser reactor typically undergo a sudden change in direction to effect a rough separation of solids from vapors prior to the solids and vapors being discharged into the surrounding disengagement vessel volume. One or more cyclone separators located either within or just external to the disengagement vessel volume then draw solids-depleted gas from the vessel volume to separate additional solids from the solids-depleted gas. The cyclones then discharge a further solids-depleted gas for processing.
Catalyst separated in these systems usually collects in a lower region of the disengagement vessel. Stripping steam typically is passed through the collected catalyst to purge hydrocarbon vapors resident in the catalyst. The purged vapors and stripping steam, collectively known as stripping gas, mix with the solids-depleted vapors present in the disengagement vessel and also exit the vessel through the cyclone separators. Examples of an open vapor path systems such as this can be found in U.S. Pat. No. 4,500,423 to Krug, et al.
The modern trend toward more reactive catalysts and higher reaction temperatures has emphasized a major deficiency in open vapor path designs similar to those discussed above. Because the desired gasoline range products are discharged from the riser reactor into a large disengagement vessel volume and reside there until drawn into a cyclone, the desired products have a sufficiently long residence time within the vessel to cause a significant decrease in yield from secondary thermal cracking.
The recognition of undesired thermal cracking problems in open vapor path systems has led to the development of systems having substantially closed vapor paths between the riser reactor and subsequent separation equipment. By maintaining closed vapor flow paths instead of discharging cracked products into the large disengagement vessel volume, the residence time of the products at thermal cracking temperatures can be greatly decreased. Examples of such closed systems can be found in U.S. Pat. Nos. 4,623,446, 4,654,060 and 4,909,993 to Haddad, et al.
Unfortunately, closed vapor path systems generally are unable to damp pressure and catalyst surges known to occur in catalytic cracking riser reactors. These surges result from equipment malfunctions, vaporization of water present in reactor feedstock or from system pressure upsets. Because the surges are not damped into a larger disengagement vessel volume, the surges propagate through the catalyst separation equipment located downstream of the reactor. The propagated surges significantly reduce the efficiency of catalyst separation equipment such as cyclone separators and therefore cause unwanted catalyst to appear in the gasoline range product stream.
Efforts have been made to obtain the surge damping advantages of an open vapor path system while at the same time maintaining an essentially closed vapor path for cracked products. In these compromise systems, riser reactor effluent follows a closed vapor path to a cyclone separator having a bottom opening into a surrounding disengagement vessel volume. One such design can be found in U.S. Pat. No. 4,478,708 to Farnsworth. Farnsworth discloses that solids can be cyclonically separated in an open-bottomed cyclone while still providing an essentially closed path through the cyclone for gaseous reaction products. Unless a pressure surge occurs, the lower pressure downstream of the open-bottomed cyclone allows gases leaving the riser reactor to follow an essentially closed path through the cyclone to the secondary catalyst separation equipment. When a pressure surge occurs, the surge pressure overcomes the effect of the lower downstream system pressure, causing the surge to be damped into the disengagement vessel volume through the open cyclone bottom.
Although Farnsworth's system represents an improvement over the designs previously discussed, his use of open-bottomed cyclones compromises the initial separation of solids and gases. Solids separated by Farnsworth's open-bottomed cyclones collect near the bottom of the disengagement vessel and are steam stripped as in the other open vapor path systems already described. Because the relatively low pressure present in Farnsworth's open-bottomed cyclones draws in a flow of stripping gas countercurrent to the generally downward flow of separated catalyst, his design permits reentrainment of solids in the upwardly moving stripping gas, thereby degrading the separation efficiency of the open-bottomed cyclone.
The limitations of the foregoing designs suggest what is needed is a solids separation system particularly suited to catalytic cracking operations. Ideally, the system provides a short, closed vapor path out of the system for vapors exiting the riser reactor so that secondary thermal cracking can be minimized. Additionally, the system preferably provides for rapid separation of spent catalyst from the reaction mixture while minimizing reentrainment of separated catalyst in the cracked vapors. Finally, the system must accommodate pressure and catalyst surges without suffering serious degradation of separation efficiency.
Various modification of the systems already discussed have been proposed to accomplish one or more of the above objects.
U.S. Pat. No. 4,581,205 to Schatz discloses that the effects of pressure surges in a catalytic cracking solids disengagement system might be mitigated by using a passageway having one or more pressure-actuated trickle valves opening from an otherwise generally closed system. Schatz' design adds an undesired degree of mechanical complexity to the system and raises questions about the reliability of the required mechanical components under the severe catalytic cracking operating conditions.
U.S. Pat. No. 4,482,451 to Kemp discloses a design in which the catalyst-vapor mixture exiting the riser is downwardly swirled as it passes through curved discharge arms to effect an initial separation of catalyst and vapors. An optional open-bottomed cylindrical shroud having a generally closed top with a discharge port encircles the discharge arms to provide a volumetrically large but generally closed vapor path for vapors exiting the arm and passing upward through the discharge port. This design appears to suffer from the same deficiencies as open-bottomed cyclone designs generally and would seem to substantially increase vapor residence time over that of a generally closed system.
U.S. Pat. No. 4,572,780 to Owen discloses a closed vapor path design having a plurality of blades located at the upper end of a riser reactor. The blades spin the catalyst and vapor exiting the riser into a deflector located just above the reactor. The catalyst particles are downwardly deflected off the deflector to effect a rapid separation of catalyst and vapors. Vapors pass vertically upward through a riser located over the deflector and into a closed cyclone separator. While the design provides a relatively short, direct, vertical path from the riser reactor, the design appears to be subject to an inability to mitigate the pressure surge problems generally associated with closed vapor path systems.
Although each of the above designs attempts to provide an improved method for quickly separating solids from a mixture of solids and gases, none of these designs provides a mechanically simple, generally closed vapor path system which simultaneously can separate solids, damp pressure surges from the riser reactor and provide for the minimal hydrocarbon vapor residence times required to minimize undersired thermal cracking.